Known master cylinders include a primary piston and a secondary piston, generally made of aluminum and both installed in series in an axial bore hole of a brake master-cylinder body, generally made of machined aluminum. Such master cylinders are described in document FR 2,827,244. A push rod in a cavity of the primary piston serves to actuate movement of the primary piston, which pressurizes a primary pressure chamber, the secondary piston pressurizing a secondary pressure chamber. Primary and secondary springs push the pistons in the direction opposite the movement, thereby ensuring the increase in pressure.
The bore hole of the master cylinder is fed brake fluid from two feed holes, also referred to as drill holes, connected to the brake fluid reservoir in order to feed the primary and secondary pressure chambers. These drill holes emerge in annular chambers, annular seals known as “cups” being provided via the pistons on either side of the annular chambers.
Brake fluid is supplied to the pressure chambers when the pistons are at rest in the position shown in FIG. 1. Supply occurs by means of supply holes in the piston walls, and the supply access holes and the annular chambers are arranged to communicate with the interior of the primary and secondary pistons, to feed those chambers, respectively, through the supply holes in the walls of the pistons. The axis of those holes is generally perpendicular to the axis of the piston. Whenever the pistons advance (direction of arrow (d) in FIG. 1), the piston holes cross the seals, isolating the supply chambers and enabling the establishment of brake pressure in the primary and secondary pressure chambers.
The master cylinder assembly is mounted on a brake assist servomotor.
When the pistons are moved in the direction of arrow (d) by the push rod, cup 4 isolates the primary pressure chamber from the primary drill hole and cup 6 isolates the secondary pressure chamber from its respective drill hole. When the force of the push rod is relaxed, the volume of brake fluid accumulated in the brakes and the springs of the master cylinder push the pistons into rest position. At times, when the push rod is rapidly released, the brake fluid contained in the pressure chambers of the master cylinder can drop below atmospheric pressure due to the action of the springs, which push the pistons more rapidly than the arrival of brake fluid in the master cylinder. When the pistons reach rest position, communication between the reservoir at atmospheric pressure and the chambers of the master cylinder is directly established, thereby producing a sudden, rapid surge or flow of brake fluid, which generates noise in the master cylinder, known as a “fluid hammer.”
The noises caused by the fluid hammer can be more or less attenuated depending on their location in the vehicle.